It is well known in the art that certain materials called thermoluminescent phosphors can be irradiated with high energy radiation, and then subsequently stimulated using heat, to produce a luminescent emission. Thermoluminescent phosphors are in widespread use in radiation dosimeters used to measure the amount of incident radiation to which people, animals, plants and other things are exposed. Thermoluminescent dosimeters are widely used by workers in the nuclear industries to provide a constant monitor for measuring exposure to radiation.
Thermoluminescent phosphors are excited by energetic radiation such as ultraviolet, X-ray, gamma, and other forms of radiation. Such ionizing radiation causes electrons within the thermoluminescent material to become highly energized. The nature of thermoluminescent materials cause these high energy electrons to be trapped at relatively stable higher energy levels. The electrons stay at these higher energy levels until additional energy, usually in the form of heat, is supplied which releases the trapped electrons thereby allowing them to fall back to a lower energy state. The return of the electrons to a lower energy state causes a release of energy primarily in the form of visible light which is ordinarily termed a luminescent emission.
The use of thermoluminescent phosphors in personal dosimeters has led to demand for a large number of dosimeters which must be read on a routine basis in order to monitor the possible exposure of persons or other objects to ionizing radiation. Because of the substantial numbers, the job of reading dosimeters becomes time-consuming and costly.
There are four commonly known methods of heating thermoluminescent material in order to release the trapped electrons and provide the luminescent emission which is measured as an indication of the amount of ionizing radiation to which the dosimeter was exposed. The first and most common method for heating thermoluminescent phosphors is by contact heating. The second method is heating using a hot gas stream which is impinged upon the phosphor. The third method uses radiant energy in the form of infrared beams which heat the luminescent phosphor. The fourth method uses infrared laser beams to provide the necessary heat for luminescent emission.
Contact heating is the most widespread, but is also the most time-consuming and unreliable. Conventional contact heating occurs using a hot finger or contacting probe which produces a highly non-uniform temperature distribution in the phosphor being heated. This is particularly true when thin layer or film type dosimeter configurations are used. Contact heating has also been found unacceptable for reading dosimeters used to measure relatively low energy beta ray doses.
Heating of phosphors using a hot gas stream has proven to be faster and more uniform than contact heating. Unfortunately, this method of heatng requires rather large scale heat exchanger equipment and is fairly costly to implement.
Heating with infrared beams has provided improved results over contact and hot gas heating. One example of such a thermoluminescent phosphor reading apparatus is shown in U.S. Pat. No. 4,204,119 to Yasuno et al. The Yasuno patent shows an apparatus using an infrared emitting incandescent lamp which rapidly heats the back surface of a substrate. A small amount of thermoluminescent powder is attached to the substrate on the opposite side from the surface exposed to the infrared beam. Yasuno thus shows a configuration where the substrate is heated directly, and through conduction the heat is transferred to the thermoluminescent powder.
U.S. Pat. No. 3,531,641 to Weissenberg teaches the manufacture of thermoluminescent dosimeters containing thermoluminescent phosphors held in a synthetic material. The synthetic material must be capable of surviving the heat and heating rays since it is used as a conductor of heat to the thermoluminescent phosphor which is contained therein.
The dosimeters and heating method described by Yasuno et al and Weissenberg have provided useful technology, but unfortunately are limited and have been found not entirely satisfactory for thermoluminescent dosimetry used in medical research, radiation therapy, and personal and environmental monitoring. Such applications preferably use or require very small dosimeters using in many cases less than one milligram of thermoluminescent phosphor in the form of a small discrete dot or very thin layer. In the case of thin layer dosimeters, it is desirable to have less than ten milligrams of thermoluminescent phosphor per square centimeter of dosimeter area. Using such small amounts of thermoluminescent material requires that the material be heated in a very rapid manner in order to provide sufficient luminescent energy emission so that detectable levels of emission can be measured without being obscured by the electronic noise in the measuring equipment. Rapid heating of such small amounts of luminescent phosphor to about 400.degree. C. is severely slowed when a substrate material must be initially heated in order to conduct heat to the thermoluminescent phosphor.
Heating of thermoluminescent phosphors by heat conduction through a substrate also severely limits the rate at which the material can be heated for another reason. If too much heat is applied very rapidly, the substrate material itself tends to incandesce thereby creating luminous emissions which are sensed by the luminescent emission detection equipment and translated into an erroneous reading of the dosimeter. Accordingly, the time required for reading thermoluminescent phosphors using conduction techniques has been severely limited and the best known times are approximately one half second.
U.S. Pat. No. 3,729,630 to Yamashita et al discloses a thermoluminescent readout instrument utilizing an infrared laser source which is used to heat a thermoluminescent dosimeter element. Instruments constructed according to Yamashita et al have been found lacking in that the luminescent emission glow curve resulting from laser stimulation does not provide the chracteristic glow peak which is desirable for easy, accurate determination of the radiation levels to which the dosimeter was exposed.
The current invention has identified that nonuniformity in the laser beam power density and also instabilities over time in the power output of the laser contribute to a lack of speed and accuracy in reading thermoluminescent phosphors. The nonuniformity in laser beam power appears to be a characteristic of carbon dioxide lasers used in this type of application. Carbon dioxide lasers typically produce a laser beam exhibiting a Gaussian or bell-shaped curve when power or intensity is graphically plotted as a function of beam cross-sectional position. This nonuniform beam power profile causes localized intense heating at the center of the beam because of the substantially higher amounts of power being released thereat. This nonuniform beam power profile causes temperatures to vary greatly, even across the small distances from the center to the outside of the laser beam.
Instabilities in the power output of the laser with time have also been identified in the current invention as a contributing factor making the resulting glow curves from the luminescent phosphors less capable of accurate interpretation. It has been previously known that laser power can be made more constant over time using temperature stabilizing devices, or closed loop controlled piezoelectric pushers which move the laser mirrors relative to one another in quick response to the thermally induced motions of the laser components. The temperature stabilization techniques for stabilizing laser power output are not totally effective, in part because of the very slow response time. The piezoelectric pushers are very expensive and accordingly have not been widely used.
It is an object of this invention to provide thermoluminescent phosphor reading apparatus which can very rapidly stimulate thermoluminescent phosphors in a manner that allows accurate and reliable measurement of the resulting luminescent emissions.
It is another object of this invention to provide methods by which thermoluminescent phosphors can be very rapidly stimulated using a laser source adapted to provide a uniform beam profile to produce very accurate and repeatable luminescent emissions which are indicative of the amount of ionizing radiation to which the original thermoluminescent material was exposed.
These and other objects and advantages of this invention will be apparent from the description given herein.